Zoom lens system and image pickup apparatus including the same

ABSTRACT

A compact zoom lens system having excellent optical performance. The zoom lens system includes a first lens unit of negative optical power, a second lens unit of positive optical power, a third lens unit of positive optical power, and an F-number defining member. The second lens unit is disposed on the image side of the first lens unit and includes a lens element disposed closest to the object side. The lens element has a rim, a surface on the object side and having a vertex, and an intersection point defined by an intersection between the surface and the rim. The third lens unit is disposed on the image side of the second lens unit. The F-number defining member is disposed along the optical axis between the vertex and the intersection point. Spaces between the first, second and third lens units vary during zooming.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system suitable for use inan image-capturing optical system, such as a digital still camera and avideo camera.

2. Description of the Related Art

With recent developments of sophisticated image pickup apparatuses(cameras), such as video cameras and digital still cameras withsolid-state image sensors, an optical system for use in such camerasrequires a zoom lens system with a large aperture ratio including a wideangle of view.

In a camera of this type, various optical members, such as a colorcorrection filter and a low-pass filter, are arranged between the end ofthe lens group and an image pickup device. Therefore, the optical systemfor use in such a camera requires a lens system with a relatively longback focal length.

For a camera with an image pickup device for color images, an opticalsystem with good telecentricity on the image side is desired to avoidcolor shading.

Conventionally, there have been proposed a variety of two-unit zoom lenssystems of a so-called short zoom type. Such a two-unit zoom lens systemincludes a first lens unit of negative refractive power and a secondlens unit of positive refractive power to perform zooming by varying thedistance therebetween. In such an optical system of a short zoom type,magnification is changed by moving the second lens unit of positiverefractive power, while compensation for positional variations of animage point associated with the changes in magnification is implementedby the first lens unit of negative refractive power. Such a two-unitzoom lens system has a zoom ratio of about 2.

To further achieve a compact lens system with a zoom ratio of as high as2 or above, a so-called three-unit zoom lens system has also beenproposed (see, for example, Japanese Patent Publication No. 7-3507corresponding to U.S. Pat. No. 4,810,072 and Japanese Patent PublicationNo. 6-40170 corresponding to U.S. Pat. No. 4,647,160). In the three-unitzoom lens system, a third lens unit of negative or positive refractivepower is arranged on the image side of a two-unit zoom lens system.

There is a known three-unit zoom lens system having a long back focallength, good telecentricity, and a wide angle of view (see, for example,Japanese Patent Laid-Open No. 63-135913 corresponding to U.S. Pat. No.4,838,666 and Japanese Patent Laid-Open No. 7-261083).

There is also a known three-unit zoom lens system in which zooming isperformed by moving a second lens unit of positive refractive power anda third lens unit of positive refractive power, while a first lens unitof negative refractive power is fixed (see, for example, Japanese PatentLaid-Open No. 3-288113 corresponding to U.S. Pat. No. 5,270,863).

There is also a known three-unit zoom lens system with a relativelysmall number of constituent lenses, in which all lens units are movedfor zooming, and a cemented lens is effectively included in a secondlens unit to correct chromatic aberrations (see, for example, JapanesePatent Laid-Open No. 2001-272602 corresponding to U.S. Pat. No.6,498,687, Japanese Patent Laid-Open No. 2002-48975 corresponding toU.S. Patent Application Publication No. 2002008920, Japanese PatentLaid-Open No. 2003-507-2, Japanese Patent Laid-Open No. 2003-149555, andJapanese Patent Laid-Open No. 2003-149556).

There is also a known three-unit zoom lens system in which a lens ofnegative refractive power in a first lens unit has aspheric surfaces onthe object side and the image side to further reduce the number ofconstituent lenses (see, for example, Japanese Patent Laid-Open No.5-323190 corresponding to U.S. Pat. No. 5,357,374, Japanese PatentLaid-Open No. 7-174971, Japanese Patent Laid-Open No. 2002-55278corresponding to U.S. Patent Application Publication No. 2003058549, andJapanese Patent Laid-Open No. 2002-365545 corresponding to U.S. PatentApplication Publication No. 2003103157).

A three-unit zoom lens system designed for 35-mm film photos is notreadily applicable to an image pickup apparatus with a solid-state imagesensor, because its back focal length is too long and its telecentricityis insufficient for use in an image pickup apparatus with a solid-stateimage sensor.

To achieve a compact camera with a zoom lens system having a high zoomratio, a so-called barrel-retractable zoom lens system has been widelyused these days. In the barrel-retractable zoom lens system, thedistance between adjacent lens units is reduced during non-use todecrease the amount of projection of lenses from the camera body.

Generally, the length of each lens unit along the optical axis increasesas the number of lenses in each lens unit increases. Moreover, the totallens length increases as the amount of movement of each lens unit forzooming and focusing increases. If the number of lenses in each lensunit is large, or if the amount of movement of each lens unit is large,the optical system causes an increase in the lens length when retracted,and thus cannot be used in a barrel-retractable zoom lens system.

SUMMARY OF THE INVENTION

The present invention is directed to a compact zoom lens system havingexcellent optical performance and an image pickup apparatusincorporating the same.

In one aspect of the present invention, a zoom lens system includes afirst lens unit of negative optical power (the reciprocal of the focallength), a second lens unit of positive optical power, a third lens unitof positive optical power, and an F-number defining member. The secondlens unit is disposed on the image side of the first lens unit andincludes a lens element disposed closest to the object side. The lenselement includes a rim, a surface on the object side and having avertex, and an intersection point defined by an intersection between thesurface and the rim. The third lens unit is disposed on the image sideof the second lens unit. The F-number defining member is disposed alongthe optical axis between the vertex and the intersection point.

Further features and advantages of the present invention will becomeapparent from the following description of exemplary embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an optical cross-section of a zoom lens system according toa first embodiment.

FIG. 2 shows aberration curves at the wide-angle end of the zoom lenssystem according to the first embodiment.

FIG. 3 shows aberration curves at the middle zoom position of the zoomlens system according to the first embodiment.

FIG. 4 shows aberration curves at the telephoto end of the zoom lenssystem according to the first embodiment.

FIG. 5 shows an optical cross-section of a zoom lens system according toa second embodiment.

FIG. 6 shows aberration curves at the wide-angle end of the zoom lenssystem according to the second embodiment.

FIG. 7 shows aberration curves at the middle zoom position of the zoomlens system according to the second embodiment.

FIG. 8 shows aberration curves at the telephoto end of the zoom lenssystem according to the second embodiment.

FIG. 9 shows an optical cross-section of a zoom lens system according toa third embodiment.

FIG. 10 shows aberration curves at the wide-angle end of the zoom lenssystem according to the third embodiment.

FIG. 11 shows aberration curves at the middle zoom position of the zoomlens system according to the third embodiment.

FIG. 12 shows aberration curves at the telephoto end of the zoom lenssystem according to the third embodiment.

FIG. 13 shows an image pickup apparatus of the present invention.

FIG. 14 shows an F-number determining member of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of a zoom lens system of the present invention and an imagepickup apparatus including the zoom lens system will now be described.

FIG. 1 is a cross-sectional view at the wide-angle end (short focallength end) of a zoom lens system according to the first embodiment ofthe present invention. FIG. 2, FIG. 3, and FIG. 4 show aberration curvesat the wide-angle end, at the middle zoom position, and at the telephotoend (long focal length end), respectively, in the zoom lens system ofthe first embodiment. The zoom lens system of the first embodiment has azoom ratio of about 2.9 and an aperture ratio ranging from about 2.6 to4.8.

FIG. 5 is a cross-sectional view at the wide-angle end of a zoom lenssystem according to the second embodiment of the present invention. FIG.6, FIG. 7, and FIG. 8 show aberration curves at the wide-angle end, atthe middle zoom position, and at the telephoto end, respectively, in thezoom lens system of the second embodiment. The zoom lens system of thesecond embodiment has a zoom ratio of about 3.0 and an aperture ratioranging from about 2.8 to 5.1.

FIG. 9 is a cross-sectional view at the wide-angle end of a zoom lenssystem according to the third embodiment of the present invention. FIG.10, FIG. 11, and FIG. 12 show aberration curves at the wide-angle end,at the middle zoom position, and at the telephoto end, respectively, inthe zoom lens system of the third embodiment. The zoom lens system ofthe third embodiment has a zoom ratio of about 3.0 and an aperture ratioranging from about 2.7 to 5.1.

FIG. 13 shows a digital still camera with a zoom lens system of thepresent invention.

The zoom lens system in each embodiment is an image-capturing lenssystem for use in an image pickup apparatus. In each of thecross-sectional views in FIGS. 1, 5, and 9, an object (front) side is onthe left and an image (rear) side is on the right.

Each of the cross-sectional views in FIGS. 1, 5, and 9 shows a firstlens unit L1 of negative refractive power (optical power=reciprocal offocal length), a second lens unit L2 of positive refractive power, athird lens unit L3 of positive refractive power, and an F-numberdefining member SP (hereinafter also referred to as “aperture stop”)serving as an aperture stop defining (limiting) an open F-number beam.

An optical block G is equivalent to an optical filter, a faceplate, acrystal low-pass filter, an infrared cut filter, or the like. If thezoom lens system is used as an image-capturing system for a video cameraor a digital still camera, an image plane IP is an imaging surface for asolid-state image sensor (photoelectric converter), such as acharge-coupled device (CCD) sensor and a complementary metal oxidesemiconductor (CMOS) sensor. If the zoom lens system is used as animage-capturing system for a silver salt film camera, the image plane IPis a photosensitive surface equivalent to a film surface.

In aberration curves, d and g denote a d line and a g line,respectively; and M and S denote a meridional image plane and a sagittalimage plane, respectively. A chromatic aberration of magnification isindicated by the g line.

In each embodiment, a wide-angle end and a telephoto end are zoompositions when a lens unit for varying the magnification is located atboth ends of the mechanically movable range along the optical axis.

In the zoom lens system of each embodiment, in zooming from thewide-angle end to the telephoto end, the first lens unit L1 reciprocatesin a curve convex towards the image side, the second lens unit L2 movesto the object side, and the third lens unit L3 moves to the image side.

In the zoom lens system of each embodiment, magnification is variedmainly by movement of the second lens unit L2, while compensation forthe shift of an image point associated with variations in magnificationis implemented by reciprocation of the first lens unit L1 and movementof the third lens unit L3 toward the image side.

The third lens unit L3 accommodates an increase in refractive power ofthe image-capturing system associated with the size reduction of animage pickup device. With the third lens unit L3 provided, the zoom lenssystem of each embodiment reduces the refractive power of a short zoomsystem composed of the first lens unit L1 and the second lens unit L2,and particularly prevents the occurrence of aberrations in lensesconstituting the first lens unit L1, thereby achieving excellent opticalperformance.

Moreover, the formation of a telecentric image on the image side, whichis required for an image pickup apparatus with a solid-state imagesensor or the like, is achieved by having the third lens unit L3 serveas a field lens.

The position of the F-number defining member SP of the present inventionwill now be described with reference to FIG. 14. In the second lens unitL2, as illustrated, the F-number defining member SP is located, alongthe optical axis, between a vertex G21 a and an intersection point G21b. The vertex G21 a is located on the object side of a lens G21 that isclosest, in the second lens unit L2, to the object. The intersectionpoint G21 b is the intersection between a surface S6 on the object sideof the lens G21 and a rim (edge) P6.

Since the F-number defining member SP is located in the second lens unitL2 and moved together therewith in zooming, the distance between anentrance pupil (related to the arrangement of the F-number definingmember SP) and the first lens unit L1 at the wide-angle side is reduced.This prevents an increase in the outer diameter (effective diameter) ofthe lenses constituting the first lens unit L1. Furthermore, the firstlens unit L1 and the third lens unit L3, with the F-number definingmember SP interposed therebetween, cancel various off-axis aberrations,thereby achieving excellent optical performance without increasing thenumber of constituent lenses.

In each embodiment, the first lens unit L1 includes a meniscus lens G11of negative refractive power and a meniscus lens G12 of positiverefractive power, in which each lens has a convex surface on the objectside. The second lens unit L2 includes, in order from the object side tothe image side, a cemented lens and a lens G23. The cemented lens isformed by bonding the meniscus lens G21 of positive refractive power toa meniscus lens G22 of negative refractive power. The lens G21 has aconvex surface on the object side, and the lens G22 has a concavesurface on the image side. The lens G23 of positive refractive power hasa convex surface on both the object and image sides. The third lens unitL3 includes a lens G31 of positive refractive power.

The first lens unit L1 may include two or more lenses, the second lensunit L2 may include three or more lenses, and the third lens unit L3 mayinclude one or more lenses.

In each embodiment, the F-number defining member SP is arranged asdescribed above to reduce the lens length when retracted.

In a known three-unit zoom lens system of a short zoom type, a lens stop(stop member) for defining an open F-number is disposed between a firstlens unit and a second lens unit. The lens stop is disposed on theobject side of a vertex of a lens that is closest, in the second lensunit, to the object.

Generally, in the first lens unit of a short zoom type, a positivemeniscus lens having a concave surface on the image side is disposed onthe side closest to the image side. Therefore, in making the distancebetween the first lens unit and the second lens unit shorter, for lensretraction, than that during image-capturing operation, the rim of alens that is closest, in the first lens unit, to the image sideinterferes with the lens stop. Thus, no further lens retraction can bemade, due to a gap created between a vertex on the object side of thelens and the rim of the lens.

Moreover, when the lens stop is disposed between the first lens unit andthe second lens unit, a certain distance must be maintained between thelens stop and a vertex on the object side of a lens that is closest, inthe second lens unit, to the object. This is another obstacle to thereduction of the lens length when retracted.

Thus, in each embodiment, as shown in FIG. 14, the F-number definingmember SP serving as a lens stop defining an open F-number is disposedbetween the vertex G21 a and the intersection point G21 b. Thisarrangement minimizes the distance between the first lens unit L1 andthe second lens unit L2, since no component causing interference in lensretraction is disposed therebetween.

In each embodiment, as described above, the lens structure and thearrangement of the F-number defining member (aperture stop) SP aredesigned such that both a desired refractive power arrangement and thecorrection of aberrations are implemented. A compact lens system withshort lens length when retracted can thus be achieved while excellentoptical performance is maintained.

The lens structure of each lens unit will now be described in detail.

The first lens unit L1 allows principal rays to converge, for apertureimaging, at the center of the F-number defining member SP. Variousoff-axis aberrations, such as astigmatism and distortion, tend to occurparticularly in a zoom area at the wide-angle side, due to a largeamount of refractive power of principal rays.

Thus, in each embodiment, similarly to a general wide-angle lens, thefirst lens unit L1 includes the lens G11 of negative refractive powerand the lens G12 of positive refractive power so that an increase in thediameter (effective diameter) of a lens closest to the object can beprevented.

Moreover, the meniscus lens G11 has an aspheric surface on the objectside that can increase positive refractive power around the perimeter ofthe lens and an aspheric surface on the image side that can decreasenegative refractive power around the perimeter of the lens. This notonly allows for the correction of astigmatism and distortion in abalanced manner, but also contributes to the reduced total lens size, asthe number of lenses constituting the first lens unit L1 is as small astwo.

Furthermore, to limit the occurrence of off-axis aberrations caused bythe refraction of principal rays, the lenses G11 and G12 constitutingthe first lens unit L1 are substantially spherical and concentric withcenter at the point where the optical axis intersects with the aperturestop SP.

Next, in the second lens unit L2, the meniscus lens G21 of positiverefractive power has a convex surface on the object side. This shapedecreases the angle of refraction of principal rays from the first lensunit L1, and limits the occurrence of various off-axis aberrations.

Moreover, the lens G21 provides the highest path through which on-axisrays pass, and is mainly related to the correction of sphericalaberrations and coma.

Thus, in each embodiment, the lens G21 has an aspheric surface on theobject side that can decrease positive refractive power around theperimeter of the lens, so that the spherical aberrations and coma can beproperly corrected.

The lens G22 bonded to the lens G21 has a concave surface on the imageside such that aberrations on the object side of the lens G21 can becanceled.

The third lens unit L3 is composed of the lens G31 with a convex surfaceon both the object side and the image side. The third lens unit L3serves also as a field lens to achieve telecentricity on the image side.

If the back focal length is sk′, the focal length of the third lens unitL3 is f3, and the imaging magnification of the third lens unit L3 is β3,their relationship can be expressed assk′=f 3(1−β3)provided that0<β3<1.0.

Here, when the third lens unit L3 is moved to the image side in zoomingfrom the wide-angle end to the telephoto end, the back focal length sk′is reduced and the imaging magnification β3 of the third lens unit L3increases in the zoom area on the telephoto side.

As a result, the third lens unit L3 varies the magnification incooperation with the second lens unit L2. This reduces the amount ofmovement of the second lens unit L2, saves space required for suchmovement, and thus contributes to the reduced size of the lens system.

When capturing images of a nearby object with the zoom lens systemaccording to each embodiment, excellent performance can be obtained bymoving the first lens unit L1 to the object side, or by moving the thirdlens unit L3 to the object side.

This not only prevents an increase in the diameter of a front lenscaused by focusing of the first lens unit L1 closest to the object side,but also prevents an increase in the load on an actuator caused bymoving the first lens unit L1, which is heaviest in terms of lensweight. Moreover, in zooming, the first lens unit L1 can be moved insynchronization with the second lens unit L2 in a simple manner using acam or the like. A simple mechanical structure and a higher degree ofaccuracy can thus be achieved.

Focusing may be performed only by the third lens unit L3.

For focusing using the third lens unit L3, if the third lens unit L3 ismoved to the image side in zooming from the wide-angle end to thetelephoto end, the third lens unit L3 can be placed on the image planeside at the wide-angle end, where the amount of movement of lens unitsfor focusing is large. This minimizes the total amount of movementrequired for the third lens unit L3 in zooming and focusing, andcontributes to the reduced size of the entire lens system.

In the zoom lens system according to each embodiment, at least one ofthe following conditions is satisfied to achieve excellent opticalperformance or to reduce the size of the entire lens system. Effectscorresponding to each condition are obtained.

The following condition is satisfied to reduce the lens length whenretracted:0.2<D 2 S/D 2 R<0.9   (1)where D2S is the distance (positive value) along the optical axisbetween the vertex G21 a on the object side of the lens G21 and theF-number defining member SP; and D2R is the distance along the opticalaxis between the vertex G21 a and the intersection point G21 b.

If the lower limit of the condition (1) is exceeded, the lens surface onthe image side of the positive lens G12 interferes with the lens surfaceon the object side of the positive lens G21, so that it is difficult tosufficiently reduce the lens length when retracted.

It is undesirable that the upper limit of the condition (1) be exceeded,because the increased distance between the F-number defining member SPand the first lens unit L1 leads to an increase in the lens diameter ofthe first lens unit L1.

A range for the condition (1) is as follows:0.3<D 2 S/D 2 R<0.8   (1a)

The following condition is satisfied to reduce the total lens length andthe lens length when retracted:0.02<(D 2 S+L 1 T)/ft<0.08   (2)where D2S is the distance along the optical axis between the vertex G21a on the object side of the lens G21 and the F-number defining memberSP; L1T is the distance, at the telephoto end, between the vertex on theimage side of the lens G12 and the vertex G21 a on the object side ofthe lens G21; and ft is the focal length of the entire lens system atthe telephoto end.

Similar to the condition (1), if the lower limit of the condition (2) isexceeded, the lens surface on the image side of the positive lens G12interferes with the lens surface on the object side of the lens G21, sothat it is difficult to sufficiently reduce the length of the lenssystem when retracted.

It is undesirable that the upper limit of the condition (2) be exceeded,because the increased distance between the F-number defining member SPand the first lens unit L1 leads to an increase in the lens diameter ofthe first lens unit L1.

A range for the condition (2) is as follows:0.03<(D 2 S+L 1 T)/ft<0.07   (2a)

The following condition is satisfied to reduce the total length of thezoom lens system (optical system):−2.6<f 1 /fw<−1.6   (3)where ft is the focal length of the first lens unit L1, and fw is thefocal length of the entire optical system at the wide-angle end.

If the upper limit of the condition (3) is exceeded, the total length ofthe optical system is reduced. However, the correction of astigmatism,especially the correction of distortion for the entire zoom area becomesdifficult, due to the reduced focal length of the first lens unit L1.

It is undesirable that the lower limit of the condition (3) be exceeded,because the amount of movement of the first lens unit L1 in zoomingincreases, which causes an increase in the total length of the opticalsystem.

A range for the condition (3) is as follows:−2.4<f 1 /fw<−1.8   (3a)

The following condition is satisfied to reduce the total lens length ofthe optical system:1.2<f 2/fw<2.0   (4)where ft is the focal length of the second lens unit L2, and fw is thefocal length of the entire optical system at the wide-angle end.

It is undesirable that the upper limit of the condition (4) be exceeded,because the amount of movement of the second lens unit L2 in zoomingincreases, which causes an increase in the total length of the opticalsystem.

If the lower limit of the condition (4) is exceeded, the total length ofthe optical system is reduced. However, the correction of astigmatismfor the entire zoom area becomes difficult, due to the reduced focallength of the second lens unit L2.

A range for the condition (4) is as follows:1.3<f 2/fw<1.8   (4a)

The following condition is satisfied to reduce the total length of theoptical system and the total lens length when retracted:1.3<(DL 1 +DL 2 +DL 3)/fw<2.0   (5)where DL1 is the distance between the vertex on the object side of thelens G11 and the vertex on the image side of the lens G12; DL2 is thedistance between the vertex on the object side of the lens G21 and thevertex on the image side of the lens G23; and DL3 is the distancebetween the vertex on the object side of the lens G31 and the vertex onthe image side of the lens G31; and fw is the focal length of the entireoptical system at the wide-angle end.

It is undesirable that the upper limit of the condition (5) be exceeded,because the total lens length when retracted cannot be easily reduceddue to the increased thickness of each lens.

If the lower limit of the condition (5) is exceeded, the reducedthickness of each lens allows for a reduced total lens length whenretracted. However, the curvature of each lens must be reduced to reducethe thickness of each lens, so that the focal length of each lens unitincreases. The increase in the focal length of each lens unit causes anincrease in the amount of movement of each lens unit in zooming. Thisleads to an increase in the length of a cam tube that allows for themovement of each lens unit. As a result, even if the thickness of eachlens is reduced, the total lens length increases.

A range for the condition (5) is as follows:1.5<(DL 1 +DL 2 +DL 3)/fw<1.9   (5a)

The following condition is satisfied:−1.6<f 11 /f 21<−0.8   (6)where f11 is the focal length of the lens G11, and f21 is the focallength of the lens G21.

In the condition (6), the absolute values of the focal length f11 andthe focal length f21 are substantially equal, and the same material isused for the lens G11 and the lens G21 to minimize the Petzval sum.

A range for the condition (6) is as follows:−1.5<f 11 /f21<−0.9   (6a)

Here, the following conditions is satisfied:1.65<n 1 n1.65<n 2 pwhere n1 n and n2 p are the refractive indexes of the materials of thelens G11 and the lens G21, respectively, and the following conditions issatisfied:1.75<n 1 n1.75<n 2 p

By setting each component as described above, each embodiment achieves azoom lens system particularly suitable for an image-capturing systemwith a solid-state image sensor, having a compact size with a smallnumber of constituent lenses, particularly suitable for abarrel-retractable zoom lens system, and having excellent opticalperformance with a zoom ratio of 2 to 3.

Moreover, each embodiment incorporates aspheric surfaces into lens unitsto properly define the refractive power, in particular, of the firstlens unit L1 and the second lens unit L2, thereby effectively correctingvarious off-axis aberrations, specifically, astigmatism, distortion, andspherical aberrations, when the aperture ratio increases.

Each embodiment described above is applicable to a type of zoom lenssystem in which two lens units (such as the first and second lens units,the first and third lens units, and the second and third lens units),instead of three lens units, are moved, in zooming, to vary the distancebetween adjacent lens units.

Moreover, a lens unit with small refractive power may be added to theobject side of the first lens unit L1, or/and to the image side of thethird lens unit L3.

Although the F-number defining member SP may be provided separately froma lens-holding member (lens holder) holding the lenses constituting thesecond lens unit L2, the lens-holding member may be given the functionof the F-number defining member SP.

Next, numerical examples of the present invention will now be described.In each example, i denotes the order of arrangement of lens surfacesfrom the object side, Ri denotes the radius of curvature of the i-thlens surface (i-th surface), Di denotes the lens thickness or air gapbetween the i-th surface and the (i+1)-th surface, Ni and νi denote therefractive index and Abbe number, respectively, with respect to the dline.

Two lens surfaces that are closest to the image side are glass members,such as faceplates. An aspheric shape can be expressed as follows:x=(h ² /R)/[1+{1−(1+k)(h/R)²}^(1/2) ]+Bh ⁴ +Ch ⁶ +Dh ⁸where x is the displacement, along the optical axis, relative to thesurface vertex at the height h from the optical axis; k is a conicalconstant; B, C, and D are aspheric coefficients; and R is paraxialradius of curvature.

In addition, “e-0X” means “×10^(−x)”, f denotes a focal length, FNodenotes an F number, and ω denotes a half angle of view. Therelationship between the respective conditional expressions describedabove and the numerical examples is shown in Table 1.

In numerical examples 1 to 3, the values of D5 are negative because lenssurfaces are counted in the order of the F-number defining member SP andthe lens G21 of the second lens unit L2, from the object side. In thestructure, as shown in FIG. 14, the F-number defining member (aperturestop) SP is located on the image side of the vertex G21 a by theabsolute value of D5.

NUMERICAL EXAMPLE 1

f = 5.95˜17.06 FNo = 2.84˜5.05 2ω = 61.6°˜23.5°  R1 = 27.620  D1 = 1.20N1 = 1.882997 v1 = 40.8  R2 = 4.710  D2 = 1.30  R3 = 6.882  D3 = 1.95 N2= 1.846660 v2 = 23.9  R4 = 15.357  D4 = variable  R5 = Stop  D5 = −0.50 R6 = 4.031  D6 = 1.70 N3 = 1.882997 v3 = 40.8  R7 = 14.306  D7 = 0.60N4 = 1.808095 v4 = 22.8  R8 = 3.293  D8 = 0.63  R9 = 12.095  D9 = 1.30N5 = 1.696797 v5 = 55.5 R10 = −16.863 D10 = variable R11 = 20.170 D11 =1.30 N6 = 1696797 v6 = 55.5 R12 = −73.163 D12 = variable R13 = ∞ D13 =1.90 N7 = 1.516330 v7 = 64.1 R14 = ∞ D\f 5.95 14.51 17.06  D4 11.70 2.040.91 D10 4.57 13.54 15.74 D12 2.50 1.19 1.15Aspheric Coefficients

-   -   R1 k=0.0000e+00 B=1.61680e−04 C=−2.27651e−06 D=2.40573e−08    -   R2 k=−1.59793e+00 B=1.58635e−03 C=9.56045e−07 D=2.10463e−07    -   R6 k=−2.87748e−01 B=−7.69444e−05 C=−1.29351e−06

NUMERICAL EXAMPLE 2

f = 5.80˜17.41 FNo = 2.75˜5.05 2ω = 62.9°˜23.1°  R1 = 25.230  D1 = 1.30N1 = 1.860000 v1 = 41.0  R2 = 4.416  D2 = 1.20  R3 = 6.152  D3 = 2.00 N2= 1.846660 v2 = 23.9  R4 = 11.714  D4 = variable  R5 = stop  D5 = −0.45 R6 = 4.045  D6 = 1.70 N3 = 1.860000 v3 = 41.0  R7 = 195.838  D7 = 0.60N4 = 1.805181 v4 = 25.4  R8 = 3.379  D8 = 0.64  R9 = 12.030  D9 = 1.30N5 = 1.696797 v5 = 55.5 R10 = −14.496 D10 = variable R11 = 18.026 D11 =1.25 N6 = 1.603112 v6 = 60.6 R12 = −108.204 D12 = variable R13 = ∞ D13 =1.90 N7 = 1.516330 v7 = 64.1 R14 = ∞ D\f 5.80 14.74 17.41  D4 10.79 1.800.84 D10 4.12 13.54 15.94 D12 2.63 1.35 1.25Aspheric Coefficients

-   -   R1 k=0.00000e +00 B=2.80275e−04 C=−2.88598e−06 D=1.46272e−08    -   R2 k=−1.39250e+00 B=1.85779e−03 C=1.54737e−05 D=4.32845e−07    -   R6 k=−3.31345e−01 B=−3.39723e−05 C=−6.42158e−06

NUMERICAL EXAMPLE 3

f = 6.00˜17.82 FNo = 2.65˜5.05 2 ω = 61.2°˜22.5°  R1 = 12.441  D1 = 1.20N1 = 1.882997 v1 = 40.8  R2 = 3.777  D2 = 1.30  R3 = 5.769  D3 = 1.80 N2= 1.846660 v2 = 23.9  R4 = 9.500  D4 = variable  R5 = stop  D5 = −0.45 R6 = 3.917  D6 = 1.80 N3 = 1.882997 v3 = 40.8  R7 = 102.584  D7 = 0.60N4 = 1.805181 v4 = 25.4  R8 = 3.188  D8 = 0.60  R9 = 11.097  D9 = 1.20N5 = 1.696797 v5 = 55.5 R10 = −15.021 D10 = variable R11 = 19.734 D11 =1.30 N6 = 1.696797 v6 = 55.5 R12 = −56.534 D12 = variable R13 = ∞ D13 =1.90 N7 = 1.516330 v7 = 64.1 R14 = ∞ D\f 6.00 15.07 17.82  D4 8.72 1.540.72 D10 3.91 13.71 16.19 D12 2.38 0.94 0.82Aspheric Coefficients

-   -   R1 k=0.00000e+00 B=−4.72130e−04 C=1.36909e−05 D=−1.60809e−07    -   R2 k=−1.30721e+00 B=1.37255e−03 C=1.84274e−05 D=1.09018e−06

R6 k=−3.29927e−01 B=−3.17633e−05 C=−1.88218e−05 TABLE 1 Lower UpperNumerical Numerical Numerical Condition Limit Limit Example 1 Example 2Example 3 (1) D2S 0.500 0.450 0.450 D2R 0.947 0.976 1.010 D2S/D2R 0.20.9 0.528 0.461 0.446 (2) D2S 0.500 0.450 0.450 L1T 0.415 0.385 0.268 ft17.065 17.410 17.824 (D2S + L1T)/ft 0.02 0.08 0.054 0.048 0.040 (3) f1−13.857 −12.547 −11.856 fw 5.951 5.802 6.005 f1/fw −2.6 −1.6 −2.328−2.162 −1.974 (4) f2 9.689 9.113 8.531 Fw 5.951 5.802 6.005 f2/fw 1.22.0 1.628 1.571 1.421 (5) DL1 4.450 4.500 4.300 DL2 4.229 4.237 4.197DL3 1.300 1.250 1.300 fw 5.951 5.802 6.005 (DL1 + DL2 + DL3)/fw 1.3 2.01.677 1.721 1.632 (6) f11 −6.593 −6.409 −6.568 f21 5.898 4.783 4.573f11/f21 −1.6 −0.8 −1.118 −1.340 −1.436

Next, an example of a digital still camera (image pickup apparatus), inwhich the zoom lens system of the present invention serves as animage-capturing optical system, will now be described with reference toFIG. 13.

Referring to FIG. 13, the digital still camera includes a camera body20; an image-capturing optical system 21 composed of the zoom lenssystem of the present invention; a solid-state image sensor(photoelectric converter) 22, such as a CCD sensor and a CMOS sensor,which receives light from a subject image formed by the image-capturingsystem 21; a memory 23 for recording information corresponding to asubject image photoelectrically converted by the solid-state imagesensor 22; and a finder 24 for observing a subject image produced by aliquid-crystal display panel, or the like, and formed on the solid-stateimage sensor 22.

An image pickup apparatus that is compact and provides high opticalperformance can thus be achieved by applying the zoom lens system of thepresent invention to an image pickup apparatus, such as a digital stillcamera.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed embodiments. On the contrary, the invention isintended to cover various modifications and equivalent arrangementsincluded within the spirit and scope of the appended claims. The scopeof the following claims is to be accorded the broadest interpretation soas to encompass all such modifications and equivalent structures andfunctions.

This application claims priority from Japanese Patent Application No.2004-148893 filed May 19, 2004, which is hereby incorporated byreference herein.

1. A zoom lens system having an optical axis and object and image sides,comprising: a first lens unit of negative optical power; a second lensunit of positive optical power, the second lens unit being disposed onthe image side of the first lens unit, wherein the second lens unitincludes a lens element disposed closest to the object side, the lenselement including: a rim; a surface on the object side and having avertex; and an intersection point defined by an intersection between thesurface and the rim; a third lens unit of positive optical power, thethird lens unit being disposed on the image side of the second lensunit; and an F-number defining member defining an open F-number beam,the F-number defining member being disposed along the optical axisbetween the vertex and the intersection point, wherein spaces betweenthe first, second and third lens units vary during zooming.
 2. The zoomlens system according to claim 1, wherein the second lens unit includesother lens elements, wherein the lens element includes a positive lenselement with a convex surface on the object side, and wherein the convexsurface of the lens element has an absolute value of radius of curvaturesmaller than those of the other lens elements.
 3. The zoom lens systemaccording to claim 1, wherein the first lens unit consists of, in orderfrom the object side to the image side, a negative meniscus lens with aconvex surface on the object side and a positive meniscus lens with aconcave surface on the image side.
 4. The zoom lens system according toclaim 1, wherein the second lens unit includes, in order from the objectside to the image side, a cemented lens composed of a positive and anegative lens, and a lens having convex surfaces on the object side andthe image side.
 5. The zoom lens system according to claim 1, wherein,in zooming from a wide-angle end to a telephoto end, the first lens unitmoves in a curve convex towards the image side, the second lens unitmoves to the object side, and the third lens unit moves to the imageside.
 6. The zoom lens system according to claim 1, wherein a distancealong the optical axis between the vertex to the object and the F-numberdefining member (D2S), and a distance along the optical axis between thevertex to the object and the intersection point (D2R) satisfy thefollowing condition:0.2<D 2 S/D 2 R<0.9.
 7. The zoom lens system according to claim 1,wherein a distance at a telephoto end between a vertex on the image sideof a lens element of the first lens unit that is closest to the imageside and the vertex on the object side of the lens element of the secondlens unit that is closest to the object side (L1T), a distance along theoptical axis between the vertex on the object side of the lens elementof the second lens unit and the F-number defining member (D2S), and afocal length at the telephoto end of the entire zoom lens system (ft)satisfy the following condition:0.02<(D 2 S+L 1 T)/ft<0.08.
 8. The zoom lens system according to claim1, wherein a focal length of the first lens unit (fl), and a focallength at the wide-angle end of the entire zoom lens system (fw) satisfythe following condition:−2.6<f 1 /fw<−1.6.
 9. The zoom lens system according to claim 1, whereina focal length of the second lens unit (f2), and a focal length at thewide-angle end of the entire zoom lens system (fw) satisfy the followingcondition:1.2<f 2/fw<2.0.
 10. The zoom lens system according to claim 1, wherein adistance between a vertex on the object side of a lens element of thefirst lens unit that is closest to the object side and the vertex on theimage side of the lens element of the first lens unit that is closest tothe image side (DL1), a distance between a vertex on the object side ofthe lens element of the second lens unit that is closest to the objectside and a vertex on the image side of the lens element of the secondlens unit that is closest to the image side (DL2), a distance between avertex on the object side of a lens element of the third lens unit thatis closest to the object side and a vertex on the image side of the lenselement of the third lens unit that is closest to the image side (DL3),and a focal length at the wide-angle end of the entire zoom lens system(fw) satisfy the following condition:1.3<(DL 1 +DL 2 +DL 3)/fw<2.0.
 11. The zoom lens system according toclaim 1, wherein the third lens unit moves toward the object side toshift focus from an object at infinity to an object nearby.
 12. The zoomlens system according to claim 1, wherein a focal length of a lenselement of the first lens unit that is closest to the object side (f11),and a focal length of the lens element of the second lens unit that isclosest to the object side (f21) satisfy the following condition:−1.6<f11/f21<−0.8.
 13. The zoom lens system according to claim 1,wherein the zoom lens system forms an image on a solid-state imagesensor.
 14. An image pickup apparatus comprising: the zoom lens systemaccording to claim 1; and a solid-state image sensor receiving lightfrom an image formed by the zoom lens system.